2.4: Global Climate and Biodiversity
The climate of a region describes the average atmospheric conditions (temperature and precipitation) that region experiences and how much those conditions vary across seasons and years. Climate differs from weather in that weather is the atmospheric conditions at any given moment while climate is the long-term averages, patterns, or trends. This distinction is discussed in more detail in the chapter on climate change.
Climate is heavily influenced by the shape of the Earth, the tilt of the Earth’s axis, and the pattern of the Earth’s movement around the sun. First, the Earth is a sphere, which means that the intensity of the sun’s energy varies across latitudes (Fig. 2.3.2). Near the geographic equator (0° latitude), the sun’s rays hit the Earth directly, and deliver a high amount of heat and light per unit area. At high latitudes (nearer to the poles), the sun’s rays strike the Earth at an oblique angle, and heat and light are spread across a larger area of the Earth’s surface. Thus, the spherical shape of the Earth is responsible for the overall pattern of warmer average temperatures near the equator and cooler average temperatures towards the poles (Fig 2.3.3).
Solar energy input also drives patterns of precipitation and atmospheric circulation (Fig 2.3.4). At the geographic equator, where the sun’s energy is intense, the warm air expands and rises. As it reaches the upper atmosphere, it cools. Since cooler air cannot hold as much water vapor as warm air, these cooling, condensing air masses lose much of their moisture as precipitation. These air masses move away from the equator, towards the north or south. Around 30° N and S latitude, these cool, dry air masses fall back towards the surface of the Earth. As they approach the surface, they warm and absorb moisture back out of the lower atmosphere, causing dry regions around 30° N and 30° S latitudes. These air masses then move back towards the equator, where they will warm, absorb more moisture and rise again, completing the cycle of air movement between 0° (the geographic equator) and 30° N and 30° S (Fig 2.3.4). This cycle of air movement is named a Hadley Cell . Similar cells exist from 30° to 60° latitudes (called Ferrel Cells) and from 60° to 90° latitudes (called Polar Cells), though the Hadley Cell is the strongest, since it is centered where the sun’s energy is most intense. The spherical shape of the Earth, therefore, and its influence on these atmospheric cells, drives the overall pattern of global precipitation, particularly the abundantly high precipitation near the geographic equator and extremely low precipitation at 30°N and 30°S latitudes (Fig 2.3.5)
As noted in Figure 2.3.2, the Earth’s axis is tilted approximately 23.5° from vertical. The orientation of the axis stays constant as the Earth rotates around the sun meaning that the intensity of solar radiation a particular region receives varies throughout the year, producing seasons (Fig 2.3.6). In December, the Northern Hemisphere is tilted away from the sun and therefore receives less intense solar energy, while the Southern Hemisphere is tilted towards the sun and therefore receives more intense solar energy. Consequently, December is winter in the Northern Hemisphere and summer in the Southern Hemisphere. In June, the opposite is true; the Southern Hemisphere is tilted away from the sun and the Northern Hemisphere is tilted towards the sun. Consequently, June is summer in the Northern Hemisphere and winter in the Southern Hemisphere. In between these extremes, during September and March, are the seasons of fall and spring.
The tilt of the Earth’s axis also causes changes in the day length, which are tied to the seasons; summer has longer days than winter. December 21 and June 21 are the extremes for day length, called
solstices
. On December 21st, the Northern Hemisphere has its shortest day (because it is angled away from the sun) and the Southern Hemisphere has its longest day (because it is angled towards the sun). Similarly, on June 21st, the Southern Hemisphere has its shortest day and the Northern Hemisphere has its longest day. The closer a region is to the pole, the greater the change in day length it will experience, such that regions north of the Arctic Circle (~66.5° N) or south of the Antarctic Circle (~66.5° S) fluctuate between 24-hour daylight at the summer solstice and 24-hour night in the winter solstice. Halfway between the solstices on September 21st and March 21st, day and night are of equal length, termed an
equinox
. The September equinox is autumn for the Northern Hemisphere and spring for the Southern Hemisphere. The March equinox is spring for the Northern Hemisphere and autumn for the Southern Hemisphere.
Figure \(\PageIndex{5}\): Relationship between the tilt of the Earth’s axis and its orbit around the sun. Figure created by L Gerhart-Barley with biorender.com
Note
It is a common misconception that the seasons are driven by the distance of the Earth from the sun, with warmer seasons occurring when the Earth is close to the sun and cooler seasons occurring when the Earth is far from the sun. The distance of the Earth from the sun does not determine seasons. The pattern of seasons throughout the year is driven entirely by the tilt of the Earth’s axis, which alters the intensity of solar radiation throughout the year. Warmer seasons occur when the solar energy is more intense and cooler seasons occur when solar energy is less intense.
On average, the Earth is approximately 93 million miles from the Sun. This distance does vary a small amount due to the fact that the Earth’s orbit around the sun is an ellipse, not a perfect circle, and the sun is not at the center of the ellipse. Consequently, at some parts of its orbit the Earth is closer to the sun than at other points in the orbit. The December solstice as shown in the figure above (2.4.6) is winter in the Northern Hemisphere because the solar intensity is lower, even though the Earth is closer to the sun at this part of the orbit. Similarly, the June solstice is summer in the Northern Hemisphere due to higher solar intensity, even though the Earth is further from the sun at this part of the orbit.
The Hadley Cell process illustrated in Figure 2.3.4 is centered where the sun’s energy is most intense, termed the thermal equator . The geographic equator (0° latitude) does not move; however, the tilt of the Earth’s axis and the Earth’s orbit around the sun mean that the thermal equator is sometimes at the geographic equator and sometimes north or south of it. The movement of the thermal equator follows a predictable pattern that is linked with the seasons, solstices, and equinoxes described above. Each December solstice, the thermal equator is at its southern extreme. After the December solstice, the thermal equator begins moving north. It crosses the equator around the September equinox and continues moving north, reaching its northern extreme on the June solstice. After the June solstice, it begins moving south, crossing the equator during the March equinox and again reaching its southern extreme at the December solstice. Note that Figure 2.3.6 is not to scale and that the movement of the thermal equator is not as extreme as the figure implies.
The rainfall produced by the Hadley Cell air masses as they rise, expand, and cool produces a band of rainclouds, termed the Inter-Tropical Convergence Zone (ITCZ). The ITCZ follows the thermal equator as it moves north and south throughout the year (Fig 2.3.8); however, since the ITCZ is in the upper atmosphere, it is also influenced by air currents and so does not always form a straight or solid band, and may even split into two bands in some seasons or in some areas.
All the concepts discussed in this section are linked. The spherical shape of the Earth produces differences in solar intensity, and the maximum solar intensity occurs at the thermal equator. The center of the Hadley Cell is linked to the thermal equator, and the rising air masses at the center of the Hadley Cell form the ITCZ cloud bands, which are also then located roughly over the thermal equator (with some variation due to atmospheric currents). The operation of the Hadley Cell drives regions of intense rainfall at the center of the Cell, over the thermal equator, and regions of intense dryness at 30° N and 30° S latitude. The tilt of the Earth’s axis and the orbit of the Earth around the sun cause the thermal equator (and therefore the center of the Hadley Cell and the ITCZ) to move north and south throughout the year, reaching its northern extreme during the June solstice and southern extreme at the December solstice, and being near the geographic equator (0° latitude) at the September and March equinoxes.
The global patterns of temperature (Fig 2.3.3) and precipitation (Fig 2.3.5) correlate to global patterns of biodiversity (Fig 2.3.9). Regions near the equator (which have high temperature and high precipitation) tend to have higher levels of diversity, while regions at higher latitudes (nearer to the poles) have overall lower diversity. This pattern is termed the latitudinal diversity gradient , and has been documented in many organismal groups, both terrestrial and aquatic. This gradient is discussed in more detail in the chapter on biomes.